Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 210318
Area Forecast Discussion
National Weather Service Blacksburg VA
1118 PM EDT Wed Jul 20 2016
High pressure was over the mid Atlantic region today and will
remain over the area through Thursday night. Two weak cold fronts
will approach the area later this week into early next week. The
first front crosses the Ohio valley and northeast United States
on Friday night and Saturday, and the next on Monday night and
.NEAR TERM /THROUGH THURSDAY/...
As of 1115 PM EDT Wednesday...
Light rain showers persist in the
Augusta/Rockbridge/Amherst/Appomattox/Nelson county area, but as
the HRRR and NAM have suggested, but to a much less extent than
advertised by the HRRR. Since this has persisted now for several
hours and was drifting slowly south more into our CWA, have adjust
pops across the region from Amherst down to Henry/Pittsylvania
over the next few hours up to 15%. Otherwise, for the remainder of
the CWA, have left them below 15%. Air mass too stable to support
thunder, so just advertised light rain showers.
T/Td/Sky grids pretty much on track. Just a very minor tweak to
the T/Td grids to bring into line better with current obs.
As of 800 PM EDT Wednesday...
Not many changes needed to the grids at this time. Minor tweaks to
the T/Td grids to bring them in better line with current
observations. So far, a very quiet evening. However, the HRRR and
to a lesser extent the NAM suggest that showers and perhaps even
thunderstorms could develop across the southern Shenandoah Valley
down into the Piedmont of VA/NC over the next few hours. Although
there is weak wind convergence in these areas and better moisture,
this does not seem very realistic. The RAP and the larger synoptic
scale models do not suggest much at all and the low-level
environment seems a tad dry and stable for much. Nonetheless,
there is a small shower in Nelson county VA at this hour, so will
monitor. Have not adjusted pops yet until this appears that it
will become more of a reality.
As of 345 PM EDT Wednesday...
Good mixing has lowered dew points into the 50s over the mountains
this afternoon. Water vapor loop showed a pocket of mid level dry
air over the Carolinas today. Environment is dry and stable
enough to keep precipitation out of the forecast until Thursday
afternoon. Hi-res NMM guidance had a reasonable placement along
the southern Blue Ridge after noon for the most likely area where
isolated thunderstorms will develop. Temperatures will be near
normal tonight and Thursday.
.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/...
As of 330 PM EDT Wednesday...
Will be increasing maximum temperature on Friday based on rising
500mb heights and forecast 850mb temperatures around +22C. Heat
index values will be at or just above 105 in the far eastern
county warning area on Saturday afternoon.
A cold front will move gradually south through Ohio and Pennsylvania
on Friday night and may continue to drift into Virginia on Saturday.
While the front may not be much of a change of air mass it will be
an area of better low level convergence and deeper moisture and when
combined with the upper support will result in an increases
probability of showers and thunderstorms. Even without the boundary
passing south of the forecast area, it will be close enough that
convergence along the boundary or along outflow boundaries will aide
in storm initiation. Precipitable water values are back up over 1.0
inch on Friday afternoon then closer to 1.75 inches on Saturday.
.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 230 PM EDT Wednesday...
Upper ridge will extend from coast to coast Sunday through
Wednesday. In the northern stream the next synoptic scale wave
crosses through the northeast United States on Tuesday. Models were
similar bringing moisture back into the region on Monday and Tuesday
ahead of the front. Therefore this will be the time frame with the
highest probability of showers and thunderstorms.
.AVIATION /03Z THURSDAY THROUGH MONDAY/...
As of 800 PM EDT Wednesday...
VFR conditions expected to prevail through much of the TAF valid
period. Cannot completely rule out some showers or perhaps even a
thunderstorm through the evening along and east of the Blue Ridge
per HRRR and NCEP 4km WRF, but certainly not enough confidence to
include in the TAFs at this point. Otherwise, the only concern
through the TAF valid period is late night/early morning fog at
the usual sites. Given less rain around the area than yesterday
and a slightly drier surface air mass in place, confidence in
patchy dense fog at the TAF sites is less than normal at this
point. Have not made any drastic changes to what was earlier
advertised. Outside the late night/early morning fog, cigs and
vsbys will be VFR through the TAF valid period with just sct cu
and sct ci. winds will be light and variable through much of the
TAF valid period, favoring a S-SE direction at 4-6kts Thu.
High confidence in cigs through the TAF valid period.
Medium to high confidence in vsbys through the TAF valid period.
Medium confidence in wind direction, high confidence in wind
speed through the TAF valid period.
Extended aviation discussion...
Strong high pressure aloft centered across the Midwest will
attempt to build eastward into the weekend, then retreat back to
the west early next week. Our region will remain on the eastern
periphery of the upper high and thus subject to weak disturbances
in northwest flow aloft tracking around the Great Lakes and into
the Mid-Atlantic. Moisture and instability will be sufficient for
isolated pop up thunderstorms, but a slightly better coverage will
be possible Friday into Saturday with a weak front/short wave
moving into the Mid-Atlantic with a better threat for convection
early next week as another front moves into the area, stalls, and
lingers over the region for several days. Hot and humid conditions
east of the Blue Ridge this weekend which may impact density
altitude. Late night and early morning fog will be possible at the
usual sites almost every day.